Observation of the transition from the near-field to the far-field region for broadband terahertz radiation

2003 ◽  
Author(s):  
Sang-Gyu Park ◽  
M.R. Melloch ◽  
A.M. Weiner
Keyword(s):  
Terahertz Radiation ◽  
Near Field ◽  
Far Field ◽  
Field Region ◽  
Broadband Terahertz

scholarly journals Experimental observation of asymmetrical microwave jets and far-field distribution generated by a dual-material system

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
B. Varghese ◽  
O. Shramkova ◽  
P. Minard ◽  
L. Blondé ◽  
V. Drazic ◽  
...  
Keyword(s):  
Near Field ◽  
Microwave Frequency ◽  
Far Field ◽  
Material System ◽  
Field Region ◽  
Edge Diffraction ◽  
Jet Behavior ◽  
Electric Field Profile ◽  
Dual Material ◽  
Plane Wave Diffraction

AbstractIn this paper, we report the experimental and numerical investigation of plane wave diffraction by an all-dielectric dual-material cuboid. Edge diffraction by a cuboid leads to the generation of a narrow, high intensity beam in the near-field region called a photonic jet. We examine the dependence of the jet behavior and orientation on the materials and dimensions of constitutive parts in the microwave frequency domain. The possibility to shift and deviate the resultant microwave jet in the near-field region of such a structure depending on the size of constitutive parts is demonstrated numerically. Experimentally, we observe a shift in the spatial position of the jet. The experimental asymmetric electric field profile observed in the far-field region is attributed to the input of multiple edge waves generated by the dual-material cuboid. The presented results may be scaled at different frequency bands such as optical frequencies for designing nanostructures enabling the focusing and deviation functionality and creation of new optical devices which would satisfy the needs of emerging nanophotonic applications.

scholarly journals Bessel-like light beams formed by the two-component scheme consisting of an axicon and a spherical lens

2020 ◽  
Vol 56 (3) ◽  
pp. 373-383
Author(s):  
N. A. Khilo ◽  
P. I. Ropot ◽  
P. K. Petrov ◽  
V. N. Belyi
Keyword(s):  
Light Field ◽  
Near Field ◽  
Cone Angle ◽  
Longitudinal Component ◽  
Optical Scheme ◽  
Far Field ◽  
Field Region ◽  
Spherical Lens ◽  
Bessel Beams ◽  
Remote Probing

The combination in an optical scheme of rather different elements such as axicons and spherical lenses allows forming light fields that differ by a variety of properties. The simplest example of such a scheme consists of an axicon and a spherical lens spatially separated from it. Though this scheme was investigated earlier, the region of so-called secondary focusing located behind the well-known annular focus has not been studied yet. In this paper, the analytical and numerical analysis of a light field in the region of secondary focusing is conducted. The boundaries of this region are determined, and the longitudinal and transverse distribution of the light intensity is calculated. It is shown that the near field region of secondary focusing is formed in the regime of abrupt autofocusing of the annular field. It is established that in a general case the transverse intensity distribution in the far field region is a superposition of an annular field and an oscillating axialtype field. The distance between the axicon and the lens is determined when the annular component of the field practically disappears. It is shown that in this case the light field in the region of the secondary focusing is a locally Bessel light beam. The peculiarity of this beam is that its cone angle depends on the longitudinal component, namely, decreases in inverse proportion while the distance z increases. The important feature of such z-dependent Bessel beams is the absence of their transformation into annular fields, as it occurs for ordinary Bessel or Bessel-Gaussian beams in the far field region. This opens the prospect for application of z-dependent Bessel beams for optical communication in free space and remote probing, which is why such beams are perspective for application in different systems of remote probing.

Turbulence anisotropy with higher-order moments in flow through passive grid under rigid boundary influence

2020 ◽  
pp. 095440622096973
Author(s):  
Pankaj Kumar Raushan ◽  
Santosh Kumar Singh ◽  
Koustuv Debnath
Keyword(s):  
Reynolds Stress ◽  
Isotropic Turbulence ◽  
Near Field ◽  
Far Field ◽  
Field Region ◽  
Rigid Boundary ◽  
Third Order ◽  
Solidity Ratio ◽  
Eigen Values ◽  
Flow Through

The investigation presents the estimate of the degree of deviation from the isotropic turbulence in terms of Reynolds stress tensor for grid generated turbulence under the influence of bottom boundary. The turbulence triangle, Eigen values, and the invariant functions are presented at near and far field regions of the grids with different solidity ratio. In addition, the work also deals with the analysis based on third-order moments of the velocity fluctuations and the ratio of momentum flux to the turbulent kinetic energy in the frequency domain. The Reynolds stress anisotropy exposes that the anisotropic invariant maps possess a closed looping trend in the near field region and an open looping trend in the far-field region of the grid. Further, to describe the physical behaviour of the velocity time-series of random fluctuating components in the stream-wise directions, the probability distribution function are estimated and interpreted.

scholarly journals Near-Field to Far-Field RCS Prediction on Arbitrary Scanning Surfaces Based on Spherical Wave Expansion

Sensors ◽  
2020 ◽  
Vol 20 (24) ◽  
pp. 7199
Author(s):  
Woobin Kim ◽  
Hyeong-Rae Im ◽  
Yeong-Hoon Noh ◽  
Ic-Pyo Hong ◽  
Hyun-Sung Tae ◽  
...  
Keyword(s):  
Near Field ◽  
Linear Equations ◽  
Spherical Wave ◽  
Scattered Wave ◽  
Far Field ◽  
Field Region ◽  
System Of Linear Equations ◽  
Wave Expansion ◽  
Field Transformation ◽  
Spherical Wave Expansion

Near-field to far-field transformation (NFFFT) is a frequently-used method in antenna and radar cross section (RCS) measurements for various applications. For weapon systems, most measurements are captured in the near-field area in an anechoic chamber, considering the security requirements for the design process and high spatial costs of far-field measurements. As the theoretical RCS value is the power ratio of the scattered wave to the incident wave in the far-field region, a scattered wave measured in the near-field region needs to be converted into field values in the far-field region. Therefore, this paper proposes a near-field to far-field transformation algorithm based on spherical wave expansion for application in near-field RCS measurement systems. If the distance and angular coordinates of each measurement point are known, the spherical wave functions in an orthogonal relationship can be calculated. If each weight is assumed to be unknown, a system of linear equations as numerous as the number of samples measured in the near electric field can be generated. In this system of linear equations, each weight value can be calculated using the iterative least squares QR-factorization method. Based on this theory, the validity of the proposed NFFFT is verified for several scatterer types, frequencies and measurement distances.

scholarly journals Different Designs of Dual-Focus Perforated Transmitarray Antenna in Near/Far-Field Region

2016 ◽  
Vol 4 (3) ◽  
pp. 25 ◽  
Author(s):  
H. Abd El-Azem Malhat ◽  
S. Zainud-Deen ◽  
W. Hassan
Keyword(s):  
Phase Distribution ◽  
Near Field ◽  
Far Field ◽  
Field Region ◽  
Radiation Characteristics ◽  
Array Configuration ◽  
Unit Cells ◽  
Single Structure ◽  
Single Sheet ◽  
Single Focus

Designs of the single-focus and multi-focused transmitarray antennas for the near-field (NF) or/and far-field (FF) applications have been introduced. Perforated dielectric single sheet is used for transmitarray design for simple configuration. Single-focus transmitarray for the far-field and the near-field are obtained. The radiation characteristics of 13×13 unit-cells transmitarray in the near-field and the far-field region have been investigated. A single structure multi-focus transmitarray is designed using the chess-board arrangement of the unit-cells elements. Multi-focus transmitarray for FF/FF, FF/NF, and NF/NF have been designed and investigated. The phase distribution and the corresponding holes radii for the first quadrant of the multi-focus transmitarray are presented. The radiation characteristics of different array configuration have been investigated and analyzed using full-wave simulator CST Microwave Studio.

scholarly journals Theoretical and Experimental Studies on the Signal Propagation in Soil for Wireless Underground Sensor Networks

Sensors ◽  
2020 ◽  
Vol 20 (9) ◽  
pp. 2580
Author(s):  
Hongwei Huang ◽  
Jingkang Shi ◽  
Fei Wang ◽  
Dongming Zhang ◽  
Dongmei Zhang
Keyword(s):  
Sensor Networks ◽  
Near Field ◽  
Path Loss ◽  
Far Field ◽  
Stage Model ◽  
Field Region ◽  
Experiment Data ◽  
Two Stage ◽  
Proposed Model ◽  
Wireless Underground Sensor Networks

Wireless Underground Sensor Networks (WUSNs), an important part of Internet of things (IoT), have many promising applications in various scenarios. Signal transmission in natural soil undergoes path loss due to absorption, radiation, reflection and scattering. The variability and dynamic of soil conditions and complexity of signal attenuation behavior make the accurate estimation of signal path loss challenging. Two existing propagation models for predicting path loss are reviewed and compared. Friis model does not consider the reflection loss and is only applicable in the far field region. The Fresnel model, only applicable in the near field region, has not considered the radiating loss and wavelength change loss. A new two stage model is proposed based on the field characteristics of antenna and considers four sources of path loss. The two stage model has a different coefficient m in the near field and far field regions. The far field distance of small size antenna is determined by three criteria: 2 D2/λ, 5 D, 1.6 λ in the proposed model. The proposed two stage model has a better agreement with the field experiment data compared to Friis and Fresnel models. The coefficient m is dependent on the soil types for the proposed model in near field region. It is observed from experiment data that the m value is in the range of 0~0.20 for sandy soils and 0.433~0.837 for clayey silt.

scholarly journals Ray tracing with multi-radiation transmitters

2012 ◽  
Vol 10 ◽  
pp. 75-78
Author(s):  
R. Brem ◽  
T. F. Eibert
Keyword(s):  
Integral Equation ◽  
Ray Tracing ◽  
Field Condition ◽  
Fast Multipole Method ◽  
Near Field ◽  
Far Field ◽  
Field Region ◽  
Equation Solving ◽  
Close Proximity ◽  
Modeling Accuracy

Abstract. A restriction in using electromagnetic ray tracing for field prediction is given by the far-field condition: the results are only valid in the far-field region of the radiator. In this paper, it will be shown how ray tracing for accurate field computation can also be applied in the near-field regions of transmitters. The reduction of required large distances between transmitter and receiver is achieved by subdividing the transmitter in smaller subtransmitters. Even for complex transmitters, e.g. antennas with objects in close proximity such as metallic carrier platforms, subtransmitter models can be very efficiently generated by using the Multilevel Fast Multipole Method (MLFMM). This well-known integral equation solving technique makes very large problems in computational electromagnetics manageable. The subtransmitters can be directly generated based on this algorithm. A simulation example will show the improved modeling accuracy and options for simplification and refinement will also be discussed.

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